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THE SKELETON AND TEETH              135
backwardly-directed processes from the apex of the spine of the axis. In the Siamang, as in Man, some of the spinous processes are directed downwards and backwards, but they are horizontal in the larger Simiidae.
As there is no ligamentum nuchse the muscles attached to the spinous processes are more numerous than in Man.
In some specimens of the Chimpanzee the transverse process of the atlas is grooved, but not perforated; and in some, Orangs the axis is grooved. But if many skeletons are examined it will be found that every species of Ape may have complete perforations of all the cervical transverse processes.
The number of thoraco-ltimbar vertebrae is sixteen to eighteen. Of these, thirteen bear ribs in the Gorilla, Chirnpanzes and Gibbon, but twelve have ribs attached to them in the Orang. In all cases seven ribs are attached to the sternum. These numbers, of course, are not absolute, for the addition or subtraction of a rib to the number which is most common is occasionally met with.
Treadgold has given the averages arrived at from the study of many Apes.
The number of lumbar, caudal and sacral vertebrae are as follows:— Gibbon ...        ...    L5, S3, C4
Sianiang          ...    L4, S3
Orang   ...        ...    L4 or 5, So or 6, O2 or 3
Chimpanzee     ...    L3 or 4, S5 or 6, 02 or 3 Gorilla...         ...    L-i or 3, S6—Coccyx rudimentary
The sacrum* is relatively wider in the Siamang than
* The sacrum is described in papers 320, 377.